1. Field of the Invention
The present invention relates to piston slide valves, and, more particularly, to a directly controlled servo valve.
2. Description of the Related Art
Piston slide valves are used in hydraulic systems as switching valves and proportional servo valves. The piston slide of the piston slide valve is being moved in a valve housing in such a manner—as a rule within a bore inside the valve housing—that flow cross sections for a fluid in the valve are more or less opened or closed, depending on the position of the piston slide relative to the valve housing in order to adjust hereby various switching positions having various flows through the valve from a pressure source to a pressure sink or respectively to a user, whereby the pressure source, the pressure sink and/or the user or several users are connected to appropriate fluid connections of the piston slide valve.
A distinction is made between pilot operated and direct controlled valves. With pilot operated valves the piston slide that is also referred to as the valve piston is moved hydraulically, whereby in order to produce the pilot forces, hydraulic valves are again used, which then as a rule must work clearly smaller and faster than the pilot operated valve—the so-called main stage. In contrast, with directly controlled valves an actuator acts mechanically or magnetically directly upon the piston slide (main piston), in order to switch the same between the various switching positions. Directly controlled valves are simpler in construction, however they are also limited in their size and positioning speed of the piston slide due to the control force that is to be applied.
One embodiment of direct controlled valves provides that the rotational movement of a motor that functions as an actuator, in particular a dynamic stepping or servo motor is converted into the desired linear movement of the piston slide. One problem with such valves is the high production cost for very precise fits of the components for the drive, in order to convert the rotational movement into the linear movement. This leads to high production costs and to the risk of jams during operation.
WO 2009/097703 A1 describes a proportional valve, wherein the drive shaft of an electric motor moves the piston slide reciprocally via a cam that engages into a recess of a yoke that is attached to the piston slide. The pivot point of the cam in the recess of the yoke is positioned on the longitudinal axis of the piston slide, inside a notch in the yoke that progresses parallel to the rotational axis of the electric motor.
U.S. Pat. No. 5,263,681 A describes a coupling between the drive shaft of an electric motor and the piston slide of a hydraulic valve which is split and designed elastically resilient and pre-tensioned against spring forces.
Patent Application US 2007/0069592 A1 describes the transfer of a rotational movement of a stepper motor via a planetary gear onto a cam that is positioned in an opening of a piston slide.
DE 25 09 523 A1 describes a switch head to operate a piston slide of a directional control valve in longitudinal direction, wherein a spherical surface engages into the cylindrical bore of a control slide and interacts with a flat surface in order to permit the piston slide to engage in certain switching positions.
U.S. Pat. No. 5,508,575 A describes an eccentric cone that is positioned on a motor drive shaft that engages in an insert that is positioned in a bore in the piston slide in order to move the piston slide by means of driving the electric motor.
WO 01/96749 A2 describes a ball eccentrically mounted on the drive shaft of an electric motor that engages into a bore, a slot or a notch of the piston slide in order to move it reciprocally.
The aforementioned arrangements have the disadvantage that between the actuating element that is mounted eccentrically on the motor drive shaft and that engages into diversely designed openings in the piston slide shaft, unfavorable surface pressures arise that vary with the displacement of the piston slide and which, on the one hand require highly precise fits during production and on the other hand favor excessive wear and tear, and jamming of the actuating element in the piston slide.
What is needed in the art is an improved piston slide valve of the aforementioned type in such a way that the valve can be produced cost effectively and simply and that the mechanism between the valve drive and the piston slide operates reliably and with little wear and tear.